The only experience I have had was with another car club. We did a RR day, one of the guys with a diesel wanted his car remapping, being nosey I stopped around to see it done. His was dyno'ed before and after, although I don't remember the gains. The price was £150, although that was 2/3 years ago. From speaking to the guy, other companies use him to write maps for them.
I will add the following day the guy who had the remap had a problem. I was nearby anyway fixing another car and took a look for him. As you can imagine the owner of the car was less than impressed. I rang the guy from the remap place, who was insistent that it wasn't the remap, but he would drive 50 miles to where the car was to check it over for the owner the following day. I thought this was a shining example of good service. As it happens, I found a vacuum end had perished and cracked, a quick swap and the car was running great again.